1. Field of the Invention
The present invention relates to spin-coating equipment for coating a substrate with a chemical solution such as a photoresist solution. More particularly, the present invention relates to a method of and apparatus for dispensing chemical solution onto a substrate in spin-coating equipment.
2. Discussion of Related Art
In general, the manufacturing of semiconductor devices includes fabrication, assembly and test processes. The fabrication processes involve processing a substrate, e.g., a polished silicon wafer, and include diffusion, thin-film formation, photolithography, and etching processes which are each well-known per se. These fabrication processes are repeatedly and selectively performed on the wafer to form electronic circuitry. Thus, the semiconductor device fabrication processes form an intermediate product which is later packaged, through an assembly process, and tested.
Among the aforementioned fabrication processes, photolithography is a process in which a circuit pattern is transferred to the wafer. First, an oxide layer is formed on a surface of the wafer to protect the surface. Then, one or more thin films are formed on the wafer by epitaxial growth or deposition processes. Next, a photosensitive chemical solution is dispensed onto the wafer, i.e., over the thin film(s). Subsequently, the wafer is rotated at a high speed so that a uniform film of the photosensitive solution is formed on the wafer. The photosensitive layer is baked to remove a volatile component of the solution and thereby leave a layer of photoresist on the wafer. Next, light is directed onto the wafer through a photo mask or reticle that bears a pattern corresponding to that which is to be formed on the wafer. Thus, the photoresist layer on the wafer is exposed to an image of the pattern borne by the photo mask. As a result, portions of the photoresist layer impinged by incident light undergo a photochemical reaction. That is, a kind of virtual image of the pattern of the photo mask is transferred to the photoresist layer.
Subsequently, the exposed layer of photoresist is developed to remove either the reacted portions of the photoresist layer (in the case of a positive photoresist) or non-reacted portions of the photoresist layer (in the case of a negative photoresist). Accordingly, the photoresist layer is patterned. Subsequently, the thin film(s) underlying the photoresist layer is/are etched with gas (dry etching) or chemicals (wet etching) using the patterned photoresist layer as an etch mask. As a result, a pattern is formed on the wafer from the thin film(s).
In a photolithography process as described above, the photoresist layer plays a very important role in the forming of the pattern on the wafer. In particular, the accuracy of critical dimensions (CD) of the pattern depends on the thickness and uniformity of the photoresist layer. In this respect, the photosensitive chemical solution must be properly and carefully dispensed onto the wafer if a photoresist layer having the desired thickness and/or uniformity is to be formed.
For instance, the photosensitive solution should not contain bubbles when it is dispensed onto the wafer. Conventional apparatus for applying a chemical solution onto a wafer employs sensors to prevent a solution containing bubbles from being dispensed onto a wafer. A first one of the sensors is installed in an upstream end of a storage tank switch valve, and an auxiliary sensor connected between the storage tank switch valve and the pipe from which the chemical solution is dispensed. In this apparatus, even if the first sensor malfunctions, the chemical solution is checked for bubbles by the auxiliary sensor.
Furthermore, apparatus for dispensing a chemical solution onto a wafer is disclosed in U.S. Pat. No. 6,332,924 B1. This patent discloses the use of a diaphragm pump and feed unit for dispensing a metered amount of chemical solution under a controlled pressure onto a wafer.
FIG. 1 is a schematic diagram of still another conventional apparatus for dispensing a chemical solution onto a wafer in spin-coating equipment. The apparatus has a canister 12 which receives a bottle 10 containing the chemical solution. An N2 purge gas supply source 11 is connected to the canister 12 by an N2 purge gas supply pipe 14 so that N2 is supplied into the canister 12 to pressurize the interior of the canister 12. The pressure forces the chemical solution out of the bottle 10 and into a chemical supply pipe 16 extending from the bottle 10.
A trap for air bubbles is disposed in the chemical supply pipe 16. The trap has the form of a tank 18 so as to receive a predetermined amount of the chemical solution flowing into the chemical supply pipe 16 from bottle 10. To this end, a photo sensor 20 having a light emitter unit and a light receptor senses whether chemical solution is present at a certain level in the tank 18. Signals generated by the photo sensor 20 are issued to a controller 28 which controls the flow of the chemical solution in the dispensing apparatus.
The bottom of the tank 18 is connected to a filter 22 via the chemical supply pipe 16. The filter 22 filters out foreign material, remnants of the chemical solution, etc. from the chemical solution. A dispensing pump 24, having two stepper motors, is connected to the filter 22 to pump the chemical solution through the apparatus.
More specifically, the controller 28 drives the dispensing pump 24 to pump chemical solution from the bottle 10 to the tank 18 when the photo sensor 20 senses that the tank 18 does not contain a predetermined amount of chemical solution. The dispensing apparatus also includes a three way valve 30 that selects a flow path of the chemical solution so that the chemical solution dispensed by the dispensing pump 24 is re-circulated or dispensed. An automatic valve 34 controls the amount of chemical solution to be dispensed under the control of the controller 28.
When chemical solution is to be applied to the wafer W, the N2 purge gas is supplied from the N2 purge gas supply source 11 to force the chemical solution stored from the bottle 10 and into the tank 18 via the chemical supply pipe 16. Then, the controller 28 opens the automatic valve 34, positions the three way valve 30 such that the nozzle 36 is in open communication with the tank 18, and turns on the dispensing pump 24. The dispensing pump 24 draws the chemical solution out of the tank 18 and through the filter 22, and forces the filtered solution out of the nozzle 36 and onto the wafer W. The automatic valve 34 remains open until a predetermined amount (for example 3.5±0.2 g) of the chemical solution pressurized by the dispensing pump 24 is dispensed from nozzle 36.
When the chemical solution is not to be dispensed, the controller 28 closes the automatic valve 34. Also, the controller 28 positions the three way valve 30 such that the chemical solution drawn from the tank 18 by the dispensing pump 28 is re-circulated to the tank 18 through a chemical recirculation pipe 32.
The conventional dispensing apparatus described above is expensive to manufacture. In particular, the dispensing pump 24 and the three way valve 30 are very expensive components. Furthermore, the dispensing pump 24 and the three way valve 30 become contaminated after a period of time (typically about three months in a conventional application) and thus, must be eventually replaced. Accordingly the maintenance costs associated with the conventional dispensing apparatus are also relatively high.
Moreover, the conventional dispensing apparatus takes a long time to empty the bottle 10 in dispensing the solution onto wafers. For example, the the conventional dispensing apparatus takes about five hours to empty a bottle of PIX3400 (1000 cc), which is a known type of photoresist solution. This is because a motor of the dispensing pump 24 must be driven slowly to prevent an excessive amount of bubbles from being formed in the solution. Thus, the conventional dispensing apparatus is rather inefficient.